This is a little bit of a dicey issue. Most communities (including Stack Exchange) don't have as established guidelines for this scenario as they do for most day-to-day situations as it simply does not come up that frequently. In the end you usually have to make a judgment call about what is best for both the community and the user involved.
When electing moderators the priority is obviously going to be on what is best for the community with the interest of candidates taking a back seat. That doesn't mean that always exposing previously private details is necessarily in the best interest of the community.
My personal solution has been to prompt the candidates themselves to reveal the information.
Your hypothetical case is very similar to one I was involved with during a recent SE election. I knew a user had a history with flags¹ that was evidence of their unsuitability for the role of moderator. Having discussed it with the an SE COMM team member who suggested revealing the issue in general terms but not giving away the actual numbers, I opted for asking them to reveal the data instead. To make it fair I actually prompted all the candidates to comment on their flag accept rates. Since most of them had nothing to hide the additional reveal of private information was not a problem and the candidate that had dirt under the rug basically gave themselves away by not lifting the rug (and arguing about whether it was important).
I've seen several other cases where users who had been suspended for one reason or another were running in moderator elections. Comments from moderators aware of the issue asking the users to explain their history on the site were quite helpful in evaluating candidates without actually giving away any details directly. The users' responses to these prompts tend to be quite informative about their character.
I don't think it's an abuse of mod powers to ask carefully chosen leading questions of somebody who is candidate to be a moderator. Passing the buck back to the candidate to reveal the relevant information themselves can fulfill the need for the community to know who they are electing without the potentially gory details necessary needing to be revealed.
¹ In this case their nomination only showing the 'helpful' count was masking the fact that their total number of flags was huge and the majority of them were declined.